A silicon nitride sintered body has been used as a wear resistant member such as a bearing ball or a roller making use of the wear resistance thereof. As a conventional sintering composition for the silicon nitride sintered body, a silicon nitride-yttrium oxide-aluminum oxide-aluminum nitride-titanium oxide-based composition or the like has been known (Patent Document 1: Japanese Patent Laid-Open No. 2001-328869). A silicon nitride sintered body having an improved sinterability (sintering property) and an excellent wear resistance has been obtained by using yttrium oxide, aluminum oxide, aluminum nitride, and titanium oxide as specific sintering aids (sintering agents).
Moreover, a silicon nitride sintered body formed by using yttrium oxide-MgAl2O4 spinel-silicon carbide-titanium oxide as the sintering aids is disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2003-34581). In Patent Document 2, the sintering temperature can be lowered to be 1600° C. or lower.
Both the sintered bodies obtained in Patent Document 1 and Patent Document 2 have had a favorable sintering property and an excellent wear resistance. On the other hand, these conventional silicon nitride sintered bodies have had a high hardness and have been hardly processable (workable) materials. It is necessary that the wear resistant member such as a bearing ball have a smooth surface having a surface roughness Ra of 0.1 μm or less as a sliding surface. A diamond abrasive grain has usually been used for surface processing of the silicon nitride sintered body; however, since the conventional silicon nitride sintered bodies are hardly processable materials, the load of polishing work has been large to become a factor of an increase in costs.